1. Field of the Invention
The present invention relates to a focus adjustment device of a cemera.
2. Related Background Art
In one of known focus adjustment devices of cameras, an exit pupil of an imaging lens is split into two areas by a focus state detecting optical system, two object images formed by light beams passing through the respective pupil areas are sensed by photo-electric transducer element arrays (for example, CCD sensor arrays), a focus status of the imaging lens is detected based on the outputs of the arrays, and the imaging lens is driven in accordance with the detection result.
In the in-focus state detection operation, a high precision focus state detection may be attained if a contrast of an object image is sufficiently high, but the focus state may not be detected when the contrast is low. In order to solve this problem, a so-called search operation in which the imaging lens is driven independently from the detection result with an expectation of rise of the contrast of the object is frequently used, because the low contrast is caused by an actual low contrast of the object or a large defocus distance of the imaging lens.
In the search operation, the imaging lens is usually driven from its nearest end to an infinite end of a distance ring of the imaging lens one reciprocation. Since this takes a substantial time for a telescope lens, the searching is effected only one and the searching is inhibited in the subsequent focus-state detection operation. If the searching is effected each time a low contrast is detected in the course of continuous focus adjustment, the searching will be continuously effected under a circumstance where the low constract state continues, and photographing may not be effected forever. Accordingly, if the low contrast is detected after one search operation, an indication such as disable to detect a focus state is displayed and the searching is inhibited.
By the above approach, the continuous searching can be prevented, but if the low contrast state disappears after the searching and the lens is ready to be driven to the in-focus position, the searching is not effected even the low contrast subsequently appears. As a result, the focus adjustment is time-consuming under such a circumstance.